The present invention relates to an optical radio transmission, more specifically, to an optical radio transmitting system including a transmitter and a receiver, where the transmitter transmits a signal over optical radio to a receiver, and a method for adjusting an optical axis of the transmitter of such optical radio transmitting system to an optical axis of the receiver of the same to establish a communication path therebetween.
A light emitting diode (LED) or a laser diode is typically used as a light emitting element in optical radio signal transmission. The laser diode is generally used to transmit signals between buildings or across a river, since it generates narrow light beams which does not spread even if travelling a long distance. However, as the light beams emitted from a laser diode are liable to cause a damage to a human body, especially eyes, it is thus undesirable to use it in a system in places such as residential buildings, office buildings, factories, etc., wherever man is present.
A LED generates light beams as shown in FIG. 12, which is of a wide directivity. Therefore, the LED is unsuitable to long-distance transmission since the light beam emitted thereby tends to spread out as the distance increases. However, a combination of such LED with a convergent lens permits increase of a transmission distance. In which case, data are frequency-modulated or phase-modulated to be broadband signals for transmission since light noise developed by lighting or illumination has mainly a spectrum in a low frequency.
The LED thus has a disadvantage that, in an optical radio transmission system in which the LED transmits signals, a diameter of the light beams increases and a power of the light beams decreases when the light beams travels for a long-distance, although employment of a convergence lens contributes to narrowing-down of the diameter to a certain degree. The spread of light beams easily causes interference when plural sets of transmission systems are used closely in parallel, or deterioration of quantity of signals when received by receivers.
In addition, the transmitting system employing LED necessitates modulation and demodulation circuits to carry out frequency-modulation or phase-modulation on signals to transmit them as broadband signals. This demands a larger scale circuit. FIGS. 13(a) through 13(c) show Manchester coded signals, FIG. 13(a) representing 0 in the Manchester coding and FIG. 13(b) representing 1. It is known that a Manchester coded signal has a spectrum distributed in a low frequency. When signals are transmitted in a from of Manchester code, every bit always has a transition at its mid-point, as shown in FIG. 13(c).
Referring to FIG. 14, a Manchester coded signal has a spectrum of which the frequency width is two times that of a Non-Return-to-Zero (NRZ) signal. The Manchester coded signals contain no direct current (DC) component but a little low frequency components. Moreover, the fact that the Manchester coded signal itself contains a clock component permits easy and complete synchronization. To process such Manchester coded signals, only simple coding and decoding circuits are required. Illustratively, FIG. 15(a) shows a circuit diagram of a coder which is formed with an exclusive OR gate to which Non-Return-to-Zero (NRZ) data and a clock signal are applied to output a Manchester coded signal. FIG. 15(b) shows a circuit diagram of a decoder which is also formed with a clock recovery circuit and exclusive 0R gate to which a clock recovered by the clock recovery circuit and a received Manchester coded signal are applied to output NRZ data.
Direct transmission of Manchester coded signals on radio waves is currently legally prohibited because its transmission band is dominated by a transfer rate of the data. There is no legal restriction on transmission on light waves, but technical problems lies in that it is impossible to completely prevent entrance of interfering light since a receiver cannot be formed with a sufficiently narrow directivity. In addition, it is difficult to employ Manchester coded signals, which contain low frequency components to a certain degree, because of concentration of noise spectrum of interfering light in a lower frequency region.
In the case of optical radio wide-band transmission or optical radio transmission through a plurality of transmission paths in the same frequency band within the same area, a narrow directivity of emitted light beams and a narrow directivity of a transmitter and a receiver are dispensable for efficient transmission. It becomes more difficult to accurately adjust an optical axis of a transmitter to an optical axis of a receiver upon settlement of a communication path as the required directivities becomes narrower. Additionally, a little deviation of the optical axis due to vibration or the like easily yields an error in the communication. In Japanese Patent Publication No. HEI 2-37997, there is disclosed a transmission system using light beams having a narrow directivity where a receiver is provided with a corner reflector which reflects incident light beams emitted from a transmitter to feed it back to the same transmitter, then the maximum level of the light beams reflected by the corner reflector is detected to adjust an optical axis of the transmitter. This system is effective, but necessitates rather complex structure, thus costly. Additionally, it is difficult to always carry out fine adjustment of the optical axis automatically in this system.